1. Field of the Invention
The present invention relates to a surface-emitting laser diode formed of a GaN series III-V nitride compound and a method for manufacturing the same, and more particularly, toga GaN series surface-emitting laser diode having a spacer for effective diffusion of holes between a p-type electrode and an active layer, and a method for manufacturing the same.
2. Description of the Related Art
As shown in FIG. 1, a general GaN series surface-emitting laser diode includes an active layer 11 of an InGaN multi-quantum well (MQW) structure, a cavity 10 having an n-AlGaN carrier barrier layer 12 under the active layer 11 and a p-AlGaN carrier barrier layer 13 on the active layer 11, each of which confines carriers to the MQW structure, and distributed Bragg reflectors (DBRs) 20 and 30 which are formed on and underneath the cavity 10, respectively, with a reflectivity of about 99%.
DBRs are classified according to materials used for the DBRs: those formed of semiconductor materials having a similar lattice constant by epitaxial growth, and those formed of dielectric materials. The former has advantages in that current can be injected through semiconductor layers and the resultant material layers have good quality. In this case, suitable semiconductor materials should have bandgap energies greater than a desired oscillation wavelength so as not to cause absorption. A greater difference in refractive index between semiconductor materials for the two DBRs is preferable. For a GaN surface-emitting laser diode, as shown in FIG. 1, suitable semiconductor materials for the DBRs 20 and 30 include GaN (for layers indicated by reference numerals 22 and 32), AlN (for layers indicated by reference numerals 21 and 31), and AlGaN. Here, AlN and AlGaN including 30% or greater Al have too large bandgap energies. For this reason, when current is injected through DBRs formed of the materials, drive voltage becomes high, causing a heat related problem. In particular, AlGaN series materials have a small difference in refractive index, and thus multiple layers, e.g., tens of layer pairs, should be deposited for DBRs to satisfy a high-reflectivity requirement for laser oscillation. Due to narrow width of a high-reflectivity region, there is a difficulty in designing surface-emitting semiconductor laser diodes. In addition, laser oscillation requirements cannot be satisfied by slight deviations in thickness of the cavity 10 or slight changes in composition of the active layer 11.
For these reasons, dielectric materials, instead of semiconductor compounds, have been widely used. In this case, current cannot be directly injected through DBRs, so a separate electrode (not shown) is required around the DBRs. The mobility of electrons is high and a doping concentration in an n-type compound semiconductor layer between an n-type electrode and an active layer can be increased. Meanwhile, the mobility of holes is smaller than that of electrons and it is impossible to increase a doping concentration in a p-type compound semiconductor layer between a p-type electrode and the active layer. Thus, there is a problem in injecting current. In addition, due to an electrode being formed around a laser output window, it is not easy to effectively diffuse holes toward the center of the laser output window and thus it is difficult to provide effective laser oscillation characteristics.